Which of the following statements about anhydrous aluminium chloride is correct?

Options

A. It can exist as `AlCl_(3)` molecule in vapour
B. it is a strong Lewis base
C. It sublimes at `180^(@)C` under vacuum
D. it is not easily hydrolysed

Answer

To determine which statement about anhydrous aluminium chloride (AlCl3) is correct, let's analyze the provided options step by step.

### Step 1: Understanding the Structure of Anhydrous Aluminium Chloride
Anhydrous aluminium chloride exists primarily as a dimer, Al2Cl6, rather than as individual AlCl3 molecules. This dimerization occurs due to the presence of coordinate covalent bonds between aluminium and chlorine atoms.

**Hint:** Remember that many metal halides, especially those of aluminium, can form dimers in the gaseous state.

### Step 2: Lewis Acid Behavior
Aluminium chloride is known to be a strong Lewis acid. This is because aluminium has an incomplete octet (it has only six electrons in its valence shell) and can accept electron pairs from Lewis bases.

**Hint:** A Lewis acid is defined as a substance that can accept an electron pair, often due to having an incomplete octet.

### Step 3: Hydrolysis of Aluminium Chloride
Anhydrous aluminium chloride readily undergoes hydrolysis when it comes in contact with water, forming aluminium hydroxide (Al(OH)3) and hydrochloric acid (HCl). This reaction indicates that AlCl3 is not stable in the presence of water and will produce an acidic solution.

**Hint:** Hydrolysis reactions often produce acids or bases, and in this case, the presence of HCl indicates an acidic solution.

### Step 4: Sublimation of Anhydrous Aluminium Chloride
Anhydrous aluminium chloride sublimes at approximately 180 degrees Celsius, but this occurs under vacuum conditions. Sublimation is the process where a solid turns directly into a gas without going through a liquid phase.

**Hint:** Sublimation is a key characteristic of certain solids, particularly those that are volatile at lower pressures.

### Step 5: Evaluating the Statements
Now, let's evaluate the provided options based on the information gathered:

1. **It can exist as AlCl3 molecule in vapor.** - **Incorrect** (It exists as a dimer, Al2Cl6).
2. **It is a strong Lewis base.** - **Incorrect** (It is a strong Lewis acid).
3. **It sublimes at 180 degrees Celsius under vacuum.** - **Correct** (This is true).
4. **It is not easily hydrolyzed.** - **Incorrect** (It undergoes hydrolysis easily).

### Conclusion
The correct statement about anhydrous aluminium chloride is: **It sublimes at 180 degrees Celsius under vacuum.**

**Final Answer:** Option 3 is the correct statement.